1. Field of the Invention
The present invention relates to a V-type engine including a secondary air supply apparatus.
2. Description of the Related Art
Usually, in a V-type engine including a secondary air supply apparatus, secondary air is taken in from an air cleaner, and the secondary air is supplied to exhaust gas passages of cylinder heads of the front and rear cylinders through a secondary air supply pipe or the like. The secondary air is supplied to the exhaust gas passage to re-burn the unburned gas, which promotes purification of an exhaust gas. Depending on a running state, sometimes back fire is caused by excessive supply of the secondary air. Therefore, in order to cut the supply of the secondary air if needed, a secondary air on-off valve, i.e., a so-called air-cut valve is disposed in a secondary air supply passage.
FIGS. 9 and 10 show arrangement examples of the secondary air pipe, the secondary air on-off valve, and the like in a conventional V-type engine. In FIGS. 9 and 10, the same component is designated by the same numeral.
In the V-type engine shown in FIG. 9, front and rear cylinders 101 and 102 are disposed in a V-shape, exhaust gas outlets 115 and 116 are formed in a front lower wall surface of a cylinder head of the front cylinder 101 and a rear lower wall surface of a cylinder head of the rear cylinder 102 respectively. An air cleaner 103 is disposed at the back of a V-bank space, and the secondary air on-off valve 105 is provided above the air cleaner 103. A secondary air source pipe 110 is connected to a secondary air inlet 105a formed at a rear end of the secondary air on-off valve 105, and the secondary air source pipe 110 is extended downward and connected to the air cleaner 103. A secondary air pipe 111 for the front cylinder 101 is connected to a secondary air outlet 105b for the front cylinder 101, formed at a front end of the secondary air on-off valve 105, and the secondary air pipe 111 for the front cylinder 101 is extended forward along an upper end face of the front cylinder 101 and connected to the exhaust gas passages of the front cylinder 101 through a lead valve 113 disposed in a front upper end portion of the front cylinder 101. On the other hand, secondary air pipe 112 for the rear cylinder 102 is connected to a secondary air outlet 105c for the rear cylinder, formed at a rear end of the secondary air on-off valve 105, and the secondary air pipe 112 for the rear cylinder 102 is extended backward along an upper end face of the rear cylinder 102 and connected to the exhaust gas passages of the rear cylinder 102 through a lead valve 114 disposed in a front upper end portion of the rear cylinder 102.
In the V-type engine shown in FIG. 10, similarly to the V-type engine of FIG. 9, the air cleaner 103 is disposed in the side portion of the V-bank space formed between the front and rear cylinders 101 and 102. On the other hand, the secondary air on-off valve 105 is disposed in the upper end face of the front cylinder 101. The secondary air source pipe 110 connected to the secondary air inlet 105a at the rear end of the secondary air on-off valve 105 is extended backward and bent downward, and the secondary air source pipe 110 is connected to the air cleaner 103. The secondary air pipe 111 for the front cylinder 101, connected to the secondary air outlet 105b at the front end of the secondary air on-off valve 105, is extended forward along the upper end face of the front cylinder 101 and connected to the exhaust gas passage of the front cylinder 101 through the lead valve 113. On the other hand, the secondary air pipe 112 for the rear cylinder 102, connected to the secondary air outlet 105c at the rear end of the secondary air on-off valve 105, passes above the V-bank space and the air cleaner 24, and the secondary air pipe 112 is extended backward along the upper end face of the rear cylinder 102 and connected to the exhaust gas passage of the rear cylinder 102 through the lead valve 114. For example, Japanese Patent Publication Laid-Open No. 2003-74319 discloses a conventional technique.
In the conventional structure shown in FIGS. 9 and 10, the secondary air on-off valve 105 is disposed above the air cleaner 103 or at the upper end of the cylinder 101, the lead valves 113 and 114 for taking the secondary air are disposed in the front end portion of the front cylinder 101 and the rear end portion of the rear cylinder 102, and the secondary air pipes 111 and 112 are provided in the upper end faces of the cylinders 101 and 102. However, in the conventional structure, unfortunately a total height of the engine is increased, a whole piping length of the secondary air pipes 111 and 112 is lengthened, and a large external piping space is required to upsize a periphery of the engine. Additionally, a fore-and-aft length is also increased in the upper portion of the engine because the lead valves 113 and 114 are disposed in the front end portion of the front cylinder 101 and the rear end portion of the rear cylinder 102.